In communications that use a low earth orbiting satellite, since the satellite moves at high speed, a Doppler frequency shift happens to a reception signal. In the Doppler shift in such a situation, the frequency of an incoming signal varies depending on the speed of the satellite. Because of this, an apparatus that reduces the error rate caused by the frequency shift is used. For example, there is a digital signal processing device that utilizes spectrum on the receiving side of the communication system.
FIG. 2 shows an example of conventional Doppler compensation system. As shown in FIG. 2, a differential mode FSK transmission signal from the satellite is received by the compensation system through a band pass filter (BPF) 1 that has a predetermined band width. The FSK transmission signal is transformed to two base band signals mutually associated with carrier wave frequency fc.
Next, these base band signals are transformed to digital data through an analog to digital convertor (ADC) 2 by a sampling frequency fs. The digital data that was sampled undergoes, to a certain degree, a frequency compensation by a digital signal processor 3, based on information from a frequency estimating device that includes a Fourier transformer (FFT) 4. Next, a signal is reproduced based on these data to obtain spectrum for each transmission symbol, by using a short time Fourier transformer (STFT) 5.
This conventional system has a relatively simple structure as described above and can compensate the Doppler shift frequencies up to, for example, a change rate of 100 Hz per second. However, this compensation range is insufficient for the modern communication system.
Generally, the amount of Doppler shift frequency is determined by a relative speed between an observer and a satellite and a carrier frequency of the communication signal. For example, if the carrier frequency is 1 GHz, the Doppler shift may be about 100 Hz/sec. If the carrier frequency becomes 10 GHz, then the Doppler shift frequency may reach to 1 kHz/sec.
In today's digital radio communication system, for example in cellular communication system, a low earth orbiting (LEO) satellite is utilized which includes a digital modulation method of quaternary phase shift keying (QPSK). In such system, a microwave band or a millimeter wave band may be utilized as a carrier frequency. Because the carrier frequency is high, comparatively fast Doppler shift fluctuations are experienced. Accordingly, these Doppler shifts need to be compensated to carry out high quality demodulation and measurement.
However, the conventional Doppler shift frequency compensation is incapable of compensating the wide range of frequency shift as may be experienced in the LEO satellite communication. In addition, the following disadvantages are associated with the conventional compensation system utilizing the short time Fourier transformer (STFT).
First, by accumulating digital data with sampling period T in the analog to digital converter ADC2, the resolution of analyzed spectrum is limited to 1/T(Hz). As a result, the frequency spectrum cannot be measured at higher resolutions than this limitation.
Second, a vector analysis frequency obtained by the FFT means 4 is discrete [n/T(Hz): n is an integer]. Therefore, if the frequency spectrum of the analog input signal does not match n/T(Hz), the amplitude value of the analyzed frequency spectrum may be adversely altered.
Third, when performing a spectrum analysis utilizing a FFT means 4, there is no significance in having an absolute phase. In other words, the absolute phase of the spectrum cannot be measured utilizing FFT means.